1. Field of the Invention
This invention relates generally to loop acquisition and, more particularly, to a system and method for determining the frequency tolerance of a synthesized signal without a frequency reference source.
2. Description of the Related Art
The use of oscillators and synthesized frequency sources are well known in communications. These frequencies are used in the generation of carrier signal and local oscillator signals, or used in the modulation and demodulation of information. The frequency tolerance of these signals is critical, and communications are degraded when the synthesized signal is out of tolerance. Conventionally, a frequency source, such as a crystal, that is highly stable with respect to temperature, initial calibration, and aging is used in the generation of the signals. Often the reference signal is baseband and must be translated up in frequency for use in the communication circuitry.
However, there are problems with the use of reference frequency circuits. The reference circuits use valuable board real estate and consume power, that may be critical in portable or battery operated equipment. Further, the parts can be expensive, with a premium paid for increased accuracy. In some applications the reference circuitry must be warmed up. If the warm up time is significant, a significant amount of data can be lost before the required frequency accuracy is obtained. Further, the additional parts count of the reference circuit increases the probability of circuit failure.
In some applications, the communication carrier frequency or modulation frequency may be variable, so the reference circuit must provide a plurality of reference frequencies. Thus, additional crystals may be required, or selectable loop dividers.
A so-called Bang-Bang phase detector can be used to acquire an input data signal without the need of a reference signal. However, the Bang-Bang phase detector cannot control the oscillator frequency with a fine degree of resolution. For example, it is difficult to use a Bang-Bang phase detector to control an oscillator sufficiently to meet synchronous optical network (SONET) standards. The accuracy of the oscillator remains uncertain unless a frequency reference is used.
It would be advantageous if accurate oscillator or clock frequencies could be generated without a reference frequency.
It would be advantageous if the oscillator frequency needed to receive communications could be derived from the received carrier signal or data signal.
Accordingly, the invention provides a system and method for determining when the oscillator or voltage controlled oscillator (VCO) clock frequency and the input date rate are within a specified frequency tolerance, without the use of a reference clock.
The method comprises: measuring the frequency of an oscillator signal; measuring the difference between the oscillator signal frequency and a data signal rate; reinitializing the measurement of the oscillator signal frequency in response to the frequency difference beatnote, or reset signal, between the oscillator and data signals; and, determining a sufficient tolerance (lock) between the oscillator frequency and data signal rate, in response to completing the measurement of the oscillator signal frequency.
More specifically, the oscillator frequency is measured by counting cycles of the oscillator signal, and a lock is determined between the oscillator signal frequency and data signal rate by counting a predetermined first number of cycles without an intervening beatnote occurrence. When a beatnote occurs, the count of the oscillator signal cycles is reinitialized.
Once lock is determined, the method further comprises: determining an insufficient tolerance (loss of lock) between the oscillator signal frequency and the data signal rate in response to generating reset signals. However, for reasons of hysteresis, at least a predetermined second number of consecutive reset signals must be counted, without an intervening count of the first number of oscillator cycles. That is, without an intervening first number count.
Additional details of the frequency tolerance determination method, and a system for determining frequency tolerance without the use of a reference frequency are presented below.